The Democratic Republic of Congo has rich deposits of gold, diamonds, copper and other minerals that unscrupulous business people have been eager to exploit. Rebel groups have used the sale of raw materials to fund civil war, and Rwandan militias in eastern Congo are capitalizing on the sale of coltan, a crucial raw material in the manufacture of electronic devices.Now, developed countries are stepping up pressure on industry to buy exclusively "clean," or "conflict-free," raw materials.   The United Nations has suggested developing a certification scheme to stamp out the trade of "dirty" or illegally mined materials.